In multi-transmitter communications networks, channel access techniques allow multiple transmitters connected to the same physical channel to share its transmission capacity. Various such channel access techniques are known in the art. For example, in second generation communications systems according to the Global System for Mobile communications (GSM) standard, Time Division Multiple Access (TDMA) techniques are utilized to divide a specific frequency channel into individual time slots assigned to individual transmitters. In third generation communications systems, Code Division Multiple Access (CDMA) techniques divide channel access in the signal space by employing a combination of spread spectrum operations and a special coding scheme in which each transmitter is assigned an individual code. The next advance in wireless communications systems considers Orthogonal Frequency Division Multiple Access (OFDMA) techniques to achieve still higher bit rates.
One major advantage of OFDMA over other channel access techniques is its robustness in the presence of multi-path signal propagation. On the other hand, the waveform of OFDMA signals exhibits very pronounced envelope fluctuations resulting in a high Peak-to-Average Power Ratio (PAPR). Signals having a high PAPR require highly linear power amplifiers to avoid excessive inter-modulation distortion, and these power amplifiers have to be operated with a large back-off from their peak power. These demands result in a low power efficiency, which places a significant burden specifically on battery operated transmitters as utilized in mobile telephones and similar portable user equipment.
The disadvantage of a high PAPR inherent to OFDMA is to a certain extent overcome by the Single Carrier Frequency Division Multiple Access (SC-FDMA) technique, which can be regarded as a modification of the OFDMA technique. The Third Generation Partnership Project (3GPP) is considering using both OFDMA and SC-FDMA in next generation communications systems currently standardized in the Long Term Evolution (LTE) project. According to section 5 of the 3GPP Technical Specification TS 36.211 “Physical Channels and Modulation”, V8.7.0 of May 2009, SC-FDMA will be implemented in LTE user equipment for the uplink direction towards the access network. OFDMA, on the other hand, will be used in the downlink direction from the LTE access network towards the user equipment.
An exemplary realization of a conventional SC-FDMA modulator stage 10 for LTE user equipment is schematically illustrated in FIG. 1. The modulator stage 10 receives as input signal a multilevel sequence of complex-valued symbols in one of several possible modulation formats such as Binary Phase Shift Keying (BPSK) or 16 level Quadrature Amplitude Modulation (16-QAM). The modulation symbols are received in blocks containing N data symbols each. Every block of N data symbols is initially subjected to an N-point Discrete Fourier Transform (DFT) in a DFT block 12. The DFT block 12 spreads the N data symbols over M frequency points or subcarriers (N<M) to obtain a frequency domain representation of the N data symbols that is input to a mapping block 14. The mapping block 14 outputs a set of M complex-valued subcarrier amplitudes. Exactly N of these amplitudes (corresponding to the M data symbols) will be non-zero, while the remaining amplitudes have been set to zero.
The M subcarrier amplitudes output by the mapping block 14 are re-transformed by an Inverse Fast Fourier Transform (IFFT) block 16 into a time domain signal. The resulting time domain signal may then be subjected to a phase rotation to correct any phase errors introduced by the previous signal processing operations in blocks 12 to 16. Furthermore, a Cyclic Prefix (CP) will be inserted into the output signal of the IFFT block 16. The CP provides a guard-time between two sequentially transmitted symbol blocks to reduce inter-block interference caused by multi-path propagation.
Except for an omission of the DFT block 12 used to spread the bits of the input symbols over the available subcarriers, an OFDMA modulator stage has a similar configuration as the SC-FDMA modulator stage 10 shown in FIG. 1. For this reason, SC-FDMA is sometimes also interpreted as DFT-spread OFDMA.
The modulation process described above for data symbols is also applied to random access preamble symbols when generating a random access signal for the PRACH as defined in section 5.7 of TS 36.211. The random access signal is used upon an initial access in a cell for uplink synchronization and for getting attached to the network for subsequent data traffic.
The generation of the random access signal starts with generation of a Zadoff-Chu sequence having a length of either 139 or 839 complex-valued symbols (also called samples) which, when modulated onto a radio carrier, give rise to an electromagnetic signal of constant amplitude. Signals comprising cyclically shifted versions of a specific Zadoff-Chu sequence do not cross-correlate (i.e., remain orthogonal to each other) when recovered at a receiver, provided that the cyclic shift is greater than a specific threshold defined by propagation delay and multi-path delay spread. An electromagnetic signal carrying a Zadoff-Chu sequence thus has a CAZAC waveform.
Once a Zadoff-Chu sequence of the required length has been generated, the resulting random access preamble symbols are transformed by the DFT block 12 of FIG. 1 to the frequency domain. Since both 139 and 839 are prime numbers, the mathematical DFT operations cannot be simplified or reduced (using, e.g., a Fast Fourier Transform, or FFT). After the mapping block 14 has applied a mapping operation to the output of the DFT block 12, the IFFT block 16 performs an IFFT of size 4.096 (in the case of 139 samples) or 24.576 (in the case of 839 samples).
It has been found that the SC-FDMA modulator stage 10 consumes considerably more hardware resources when processing a CAZAC-based random access signal compared to the processing of conventional data signals.